1. Field of the Invention
The present invention relates generally to compensation of switch-mode power supplies, and more specifically, to a switch-mode power supply in which the compensation is auto-tuned dynamically during operation.
2. Background of the Invention
Switching power converters, referred to as switch-mode power supplies (SMPSs) are currently in widespread use for applications such as systems power supplies, AC power inverters, as well as localized power supplies such as voltage regulator modules (VRMs). In a SMPS, one or more magnetic storage elements such as inductors or transformers are energized and interrupted by a switching circuit and the stored energy is typically periodically transferred to one or more capacitive storage elements. The output voltage or output current (or an analog of the output voltage/current) of the SMPS is sensed by a sensing circuit and used to control the switching circuit so that voltage or current regulation is provided over a variety of input voltage, output load and temperature variation ranges.
A compensation circuit or “compensator” is provided in the feedback and/or feed-forward paths of the converter between the sensing circuit and the switching circuit and sets the control response of SMPS to the sensed output voltage and/or current. The compensator modifies the closed-loop response of the converter response to ensure that the converter is stable and ensure other operating conditions. The crossover bandwidth is the bandwidth at which the converter loop gain becomes unity, and is a function of the reactance and resistance of the above-mentioned inductive and capacitive storage element(s), as well as the open loop gain of the converter circuits and the compensator. The crossover bandwidth is set to a frequency low enough that the phase shift around the converter loop is less than 180 degrees by a phase margin.
Since the reactance and resistance values of the capacitors and inductors used in SMPS can vary widely both from device-to-device and over temperature and device aging, a very conservative approach to compensation must typically be taken. Device-to-device variations can be compensated-for by production tuning, but at considerable cost and potentially high rejection rates if a conservative design is not chosen. Such conservative designs typically require capacitors having at least 40% greater capacitance than would be necessary for an optimally-tuned SMPS. The capacitors are typically the most expensive components of the SMPS and also one of the largest space and weight consumers, particularly for a high-frequency SMPS, in which the transformers and/or inductors can be made very small.
A conservative design also imposes a limitation on the ability of the SMPS to prevent voltage transients at the output of the power supply that are either due to changing load conditions, or transients at the input of the SMPS. It is possible to decrease the magnitude of voltage transients at the output of an SMPS by increasing the crossover frequency up to a limit known as the “critical frequency” or “critical bandwidth”, above which the transients are not reduced by increasing the bandwidth of the loop. The equivalent series resistance (esr) of the output capacitor(s), as well as the capacitance is determinative of the critical bandwidth of a converter, as the capacitor receives all of the inductor current if the load current is suddenly reduced and the additional current from the inductor causes a voltage increase due to the capacitor impedance. Therefore, in order to either meet a predetermined transient voltage specification, or to provide optimum transient performance, it is desirable to provide an SMPS loop response that approaches the critical bandwidth.
However, even if a particular set of storage element parameters is known for an off-the-shelf SMPS design, the connected load will change the characteristics of SMPS operation so that an ideal response is not possible for all applications. For example, when an SMPS is connected to digital equipment, the power supply distribution buses typically have large amounts of capacitance provided for decoupling and local energy storage to reduce the amplitude of transient voltage due to digital switching. The amount of capacitance will vary from application to application and the esr of the external capacitance and for some capacitor types (e.g., aluminum electrolytic capacitors) the capacitance itself will vary widely with operating temperature.
The design of such an “ideal” converter is further exacerbated for manufacturers of controller integrated circuits (Ics) intended for use in off-the-shelf SMPSs or use by other manufacturers in on-board SMPS designs that form part of a larger sub-system. The controller ICs must be able to implement SMPS compensators not only in varying applications, but for SMPS designs with wide ranges of storage element reactances and resistances.
Theoretically, a digital or analog compensator could be provided with tuning control to adjust the feedback response applied between the sensing circuit(s) and the switching circuit of an SMPS, so that the above variations can be taken into account. In particular, digital compensators, which are essentially digital filters, integrators and or integator/differentiator circuits, can implement almost completely arbitrary frequency and phase responses. However, the response of the converter must be obtained in order to determine the appropriate compensator and therefore the above-mentioned parameters of the converter must be extracted or the converter response otherwise measured, in order to adjust the compensator response.
Auto-compensation techniques have been attempted at converter start-up that measure the response of the converter by injecting a signal such as a pseudo-random noise signal. However, such techniques do not measure the converter response under actual loading and operating conditions and cannot be used during actual converter operation. Converter output noise, electromagnetic interference (EMI) and transient voltage specifications will typically not permit such signal/noise injection during operation, and differentiating between the converter response due to the injected signal versus the behavior of the SMPS line or load conditions is at least problematic, if not impossible. Further, once compensation has been chosen, the SMPS performance still varies with temperature and line/load conditions, and therefore a compensator design must still be chosen in a manner sufficiently conservative to account for the possible future variations in the above-mentioned conditions, as well as for production component tolerances.
Therefore, it would be desirable to provide a method and system for auto-tuning an SMPS compensator during converter operation that can account for temperature, device aging, load/line condition variations and production component parameter variation. It would further be desirable to provide such a method and system that introduces little or no interference with normal converter operation. It would also be desirable to provide such a method and system that can either maintain a predetermined transient voltage specification with a less-than-typically conservative compensator design, or provide optimized bandwidth performance for a particular set of converter parameters and operating conditions.